It has been reported that TSS strains of S. aureus are less hemolytic than non-TSS strains. We have found that, although 30 isolates of toxic shock (TSS)-associated S. aureus strains were less hemolytic on rabbit, human, mouse or sheep blooid cells (rbc) when incubated in air than were 50 clinical non-TSS-associated strains, the two strain groups were equally hemolytic on all these rbc upon substitution of air containing 20-30% CO2 for air alone during incubation and of (on a solid medium) agarose (sulfur less than 0.02%) for agar. Neutralization studies with specific antiserums or isoelectric focusing (IEF) rbc overlay studies have demonstrated that, unlike non-TSS-associated strains, most TSS strains are negative for Alpha-toxin and that the CO2-dependent, agar-sensitive hemolytic activity of the TSS strains is due to a Gamma-like toxin (the only hemolytic toxin inhibited by agar) and a Delta-toxin potentiated by a second agent (possibly Gamma-toxin). Mice receiving sublethal doses of culture filtrates from TSS-associated strains developed changes in blood chemistry indicative of shock, liver, and kidney damage. A significant association between the hemolytic units on human or mouse rbc in filtrates and the extent of these changes was found. Studies will be conducted to characterize and assess the clinical significance of the unusual hemolytic activity of the TSS strains by determining: 1) the role of Gamma-, or a Gamma- Delta-human) and the biological activity of the TSS culture filtrates by use of purified toxins and specific antitoxins; 2) whether the Gamma- or Delta-toxins are modified in TSS strains as compared to non-TSS-associated strains by IEF rbc overlay and immunoblot studies; 3) association of Gamma-toxin with TSS by examining more TSS-associated (TSST-1 negative as well as positive) and more TSS non-associated strains of known source; 4) possible human host susceptibility by examining individual typed human bloods for sensitivity, and acute and convalescent sera of TSS patients for specific anti-lytic activity; 5) effect of (Alpha- negative) filtrates with and without the unusual Gamma-like hemolytic activity on the clinical parameters (temperature, weight, hematology, blood chemistry) in mice with any changes in these parameters compared with those resulting from the purified protein(s) believed to be responsible.